1. Field of the Invention
This invention relates to an optical transmission system, particularly to an optical time division multiplexing technique. This invention can be applied to, a measuring system, particularly to a temperature sensor and a pressure sensor.
2. Description of the Related Art
In an optical transmission system according to the related art, a transmitting apparatus performs TDM (Time Division Multiplexing) of a plurality of low-speed signals by processing in an electronic circuit and generates high-speed signals. A receiving apparatus or a node performs DEMUX (Demultiplexing) of the high-speed signals by processing in an electronic circuit and regenerates the low-speed signals. A transmission system with a transmission level of 10 Gb/s (bits per second) has been already realized by adopting the TDM technique in an electronic circuit. However, a processing speed of the electronic circuit is likely to become a bottle neck for a future transmission system with a larger capacity. Therefore, the time division multiplexing technique (optical TDM technique) for performing optical processing is currently under intense studies.
According to the optical TDM technique, an optical signal is processed without being converted to an electric signal. Optical pulse strings inputted from various transmission lines must be multiplexed synchronously. However, the optical pulse strings are transmitted in the transmission lines at different transmission rates depending on environmental factors such as a temperature, etc. Further, pulse positions (phases) of inputted optical pulse strings change constantly. Therefore, it is necessary to provide a method for detecting the pulse positions. The pulse positions are relative time relationships of standard clock signals with repetitive frequencies of the optical pulse strings and inputted pulses.
A technique for detecting a pulse position is disclosed by Ohteru, et. al.
A block chart in "Optical Time-Division-Multiplexer Based on Modulation Signal to Optical Modulators," B-1118 in Proceedings of the 1996 Institute of Electronics, Information and Communication Engineers (IEICE) General Conference is revised in FIG. 43.
In FIG. 43, an optical pulse string input terminal 1, an optical multiplexer 2, an optical modulator 3 for modulating an optical power level, a phase shifter 4, an oscillator 5, optical power meters 6a and 6b for detecting power levels of optical signals and a transmittancy rate detector 7 are illustrated.
In FIG. 43, an optical pulse string is inputted from the optical pulse string input terminal 1 and multiplexed into two transmission lines by the optical multiplexer 2. A first output from the optical multiplexer 2 is inputted to the first optical power meter 6a and a second output from the optical multiplexer is inputted to the optical modulator 3. A standard clock signal outputted from the oscillator 5 drives the optical modulator 3 via the phase shifter 4. An optical signal outputted from the optical modulator 3 is inputted to the second power meter 6b. The transmittancy rate detector 7 performs a comparative operation of outputs from the first and second optical power meters 6a and 6b, and detects a transmittancy rate of a pulse in the optical modulator. Since the comparative operation of the outputs from the first and second power meters 6a and 6b is performed, even if the optical power level of the inputted optical pulse string fluctuates, the transmittancy rate in the optical modulator 3 can be measured. As discussed below, the transmittancy rate of a pulse in the optical modulator is determined from a phase of a clock signal for driving the optical modulator and a position (phase) of an optical pulse string inputted to the optical modulator. Therefore, a pulse position can be known from the transmittancy rate. The phase shifter 4 is controlled manually to increase a value of the transmittancy rate. Accordingly, the phase of the inputted optical pulse and the phase of the standard clock signal can be synchronized.
An operation of FIG. 43 is discussed with reference to FIG. 44.
In FIG. 44, pulse positions (a) in an optical pulse string inputted to the optical modulator 3 are illustrated. A relation (b) of a time and a transmittancy rate in the optical modulator 3 is also shown. The relation corresponds to the clock signal which drives the optical modulator 3. A relation (c) of a time and a transmittancy rate of a pulse is also shown.
In FIG. 44, three pulse positions (a) of pulse 1, pulse 2 and pulse 3 in the optical pulse string correspond to transmittancy rates 1, 2 and 3 in (c). Since the pulse positions and the transmittancy rates correspond, the pulse positions can be known by detecting the transmittancy rates.
In technique illustrated in FIG. 43, it is assumed that a pulse position detector is provided as an error signal detecting circuit for controlling pulse positions. Therefore, it is not necessary to detect an accurate pulse position. It is only necessary to detect a sign relationship (left-or-right from position A in (b) of FIG. 44) of the detected pulse position.
However, when it is necessary to detect the pulse positions accurately, following problems arise from the technique illustrated in FIG. 43. As apparent from (c) of FIG. 44, the transmittancy rates 1, 2 and 3 correspond to pulses 1', 2' and 3' as well as pulses 1, 2 and 3. A range for detecting positions is limited to field T in (b) of FIG. 44. Since the transmittancy rate in the optical modulator corresponds to two phase shift amounts, it is difficult to optimize the phase shift amounts. As shown in (c) of FIG. 44, the relation of the transmittancy rate and the pulse position is not a straight-line but a sine function curve, a complicated operation circuit is necessary to detect the accurate pulse positions.
It is necessary to detect the accurate pulse positions to simplify a controlling circuit of the pulse positions and to perform more complicated optical processing. Detection of the accurate pulse positions is also necessary for various sensors that utilize changes of a transmission delay time in transmission lines.
Another technique for detecting a pulse position is disclosed in Japanese Unexamined Published Patent Application HEI 2-1828. FIG. 1 in HEI 2-1828 is revised in FIG. 45 for this specification.
In FIG. 45, the optical pulse string input terminal 1, an optical demultiplexer 33, a fully-optical modulator 43 for modulating an optical pulse string with an optical clock pulse and a photo detector 8 are illustrated. In FIG. 45, an optical delayer controlling circuit 34, an optical clock pulse generating circuit 44, an optical delayer 24 and a phase shift amount output terminal 10 are also illustrated. An optical signal is inputted from the optical pulse string input terminal 1 and inputted to the photo detector 8 via the fully-optical modulator 43. Then, the photo detector 8 outputs a signal to the optical clock pulse generating circuit 44, and the optical clock pulse generating circuit 44 outputs an optical clock pulse. The optical delayer 24 delays the optical clock pulse and the optical demultiplexer 33 inputs the delayed optical clock pulse to the fully-optical modulator 43. The fully-optical modulator 43 is designed to have a higher transmittancy rate when an optical signal with a higher power is inputted. Therefore, when phases of the optical signal inputted from the optical pulse string input terminal 1 and the optical clock pulse synchronize, the photo detector 8 detects a maximum optical power. When the optical delayer 24 is controlled to maximize the output from the photo detector 8, the optical clock pulse synchronizes with the inputted optical pulse. A pulse position of the inputted optical pulse string can be detected by monitoring an amount of delay for the optical delayer 24 from the phase shift amount output terminal 10.
In the technique illustrated in FIG. 45, the optical clock pulse generating circuit 44 needed to detect an accurate pulse position is complicated. In particular, the optical clock pulse generating circuit 44 needs to output an optical clock pulse synchronized with an inputted optical signal. However, it is not desirable that such a complicated optical clock pulse generating circuit 44 is provided only to detect the pulse position. Besides, even though some embodiments of the fully-optical modulator 43 are known, they are not available in a market. Further, it is difficult to produce the optical delayer 24 for controlling a longer delay time more accurately than an electric delayer (phase shifter).
An importance of realizing a short pulse generating circuit according to the optical TDM technique is discussed. The short pulse is an optical pulse with a short pulse width.
When a transmission capacity increases, an optical pulse with a shorter pulse width is required. Therefore, generation of the short pulse is important in the optical TDM technique. For example, in a transmission system with a transmission capacity of 20 Gb/s according to the optical TDM technique, an optical pulse with a pulse width of 20 ps (pico second) or less is necessary. In a transmission system with a transmission capacity of 100 Gb/s, an optical pulse with a pulse width of 4 ps or less is necessary.
One known pulse generation method uses an optical modulator which includes a pulse type gate. A technique of connecting optical modulators which include pulse type gates in multi-layers and thinning an effective gate width to generate a short pulse is disclosed in "Super High Speed Optical Technique, 2. Chapter 2," by Tatsuo Yajima, Maruzen Co.
In order to generate the short pulse by connecting the optical modulators in multi-layers, it is necessary to balance a phase of an electric signal for driving each of the optical modulators with a phase of an optical pulse inputted to each of the optical modulators.
Generally, optical amplifiers are provided between the optical modulators to compensate an insertion loss to each of the optical modulators. However, since the optical signals are transmitted at different transmission rates in the optical amplifiers and the transmission lines depending on environmental temperatures, it is difficult to balance the phases of the electric signal and the optical pulse without providing a system for absorbing a fluctuation in a delay time of the optical signal.
A technique for balancing the phases is disclosed by Tomioka, et. al.
A block chart in "A Control Method of Phase between Ultrashort Optical Pulses and Modulation Data," B-1121 in Proceedings of the 1996 Institute of Electronics, Information and Communication Engineers (IEICE) General Conference is revised for FIG. 46.
In FIG. 46, a light source 26, a first optical modulator 3a, an optical amplifier 29, a second optical modulator 3b, RF (Radio Frequency) amplifiers 28a and 28b, the phase shifter 4, and the oscillator 5 are illustrated. In FIG. 46, a 2:1 multiplexing circuit 32, a pulse pattern generator 31, the optical multiplexer 2, a modulation light output terminal 30 and an optical power meter 6 are also shown.
Operations are performed as follows.
A clock signal is outputted from the oscillator 5 and branched to the pulse pattern generator 31 and the phase shifter 4. A phase of the clock signal is shifted by the phase shifter 4 and amplified by the RF amplifier 28a. The amplified clock signal is inputted to the optical modulator 3a. The optical modulator 3a modulates an optical signal outputted from the light source 26 by the clock signal outputted from the RF amplifier 28a, and outputs an optical pulse. The optical pulse outputted from the optical modulator 3a is amplified by the optical amplifier 29 and inputted to the second optical modulator 3b. An output from the pulse pattern generator 31 is RZ (Return to Zero) encoded by the 2:1 multiplexing circuit 32, and amplified by the RF amplifier 28b. A RZ signal synchronized with the clock signal outputted from the oscillator 5 is inputted from the RF amplifier 28b to the second optical modulator 3b. The second optical modulator 3b modulates the optical pulse outputted from the optical amplifier 29 with the RZ signal outputted from the RF amplifier 28b. At this time, it is necessary that the optical pulse inputted to the optical modulator 3b and the RZ signal are synchronized.
A part of an output from the optical modulator 3b is branched by the optical multiplexer 2 and inputted to the optical power meter 6.
An operation principle is discussed with reference to FIG. 47. A relation between the phase shift amount of the phase shifter 4 and the transmittancy rate in the optical modulator 3b is shown in (b). In (b) of FIG. 47, when the optical pulse inputted to the optical modulator 3b and the RZ signal are synchronized, the phase amount is a phase shift amount 2. When the phase shift amount is too small (phase shift amount 1) or too large (phase shift amount 3), the transmittancy rate in the optical modulator 3b decreases. Therefore, a phase relation of the optical pulse inputted to the optical modulator 3b and the RZ signal can be optimized by controlling the phase shift amount of the phase shifter 4 manually to maximize an optical power level of the output from the optical power meter 6.
The technique illustrated in FIG. 46 is not intended to generate a short pulse but to generate a RZ modulated optical signal. Therefore, the optical modulator-3b is driven by the RZ signal. However, when the RZ signal for driving the optical modulator 3b is in an ideal short wave, the phase shift amount cannot be measured from the transmittancy rate in the optical modulator 3b. Further, even if it is intended to generate a proper pulse waveform, since the relation between the transmittancy rate and the pulse position is not a straight-line but a sine function curve, a change (.DELTA.P) of the transmittancy rate in the optical modulator 3b against a change (.DELTA..PHI.) of the phase shift amount becomes smaller around the optimal phase shift amount (phase shift amount 2). Hence, a control accuracy drops. Besides, since the transmittancy rates in optical modulators correspond to two phase shift amounts as in technique illustrated in FIG. 43, it is difficult to optimize the phase shift amount in the technique illustrated in FIG. 46. Further, a comparison of the outputs from the first optical power meter 6a and the second optical power meter 6b is not performed in the configuration illustrated in FIG. 46 differing from the configuration illustrated in FIG. 43. Therefore, when one of the optical power outputted from the light source 26, the transmittancy rate in the optical modulator 3a and a gain of the optical amplifier 29 is changed, it becomes impossible to measure the transmittancy rate in the optical modulator 3b.
In a long distance optical amplifying relay transmission system, it is well known that an optical S/N ratio (Optical Signal to Noise Ratio) deteriorates or fluctuates by polarization hole burning in an optical amplifying delayer and a polarization reliance loss in transmission lines. In order to improve the optical S/N ratio, polarization scramble is performed. The polarization scramble is a method for transmitting a signal by switching two kinds of independent polarizations from time to time. The polarization scramble is performed one or more times for a signal of one bit. Particularly, the polarization scramble must be performed at a speed of a signal bit rate or higher to average out a fluctuation (signal fading) of the optical S/N ratio. Generally, a Lithium Niobate optical phase modulator is used to perform polarization scramble.
Particularly, in the polarization scramble, phase modulation occurs simultaneously with polarization modulation. It is published in Japanese Unexamined Published Patent Application HEI 8-111662 that the polarization scramble is used to compensate wave-form deterioration due to dispersion of transmission lines (differences in optical transmission rates according to frequencies based on transmission line characteristics). In HEI 8-111662, a polarization scrambler must be driven by a data clock synchronized with a data fluctuation bit. Therefore, it is discussed that a significant improvement in a sign error rate in a super long distance optical amplifying relay transmission system across the ocean. Hence, when a determined relation between a phase of a driving signal of the polarization scrambler and a phase of a data is maintained, an opening of an eye can be further enlarged to an advantage for distinguishing a signal, even if a fiber dispersion (differences in transmission rates of light according to frequencies based on fiber characteristics) and an amplitude fluctuation at an input terminal by fluctuation of non-linear refractive index occur.
FIG. 43 illustrates a technique for controlling phases between the optical modulators based on optical power level. However, since two pulse positions are assumed from a transmittancy rate, it is impossible to know a direction of the phase shift only from the optical power level. Further, since the phase shifter is controlled manually, it is difficult to control the phases between the optical modulators automatically reflecting a change in a transmission line length between the optical modulators which fluctuates from time to time. Further, since the relation between the transmittancy rate and the pulse position is not a straight-line, a complicated operation circuit is necessary.
FIG. 45 illustrates another technique for detecting the pulse position. However, a complicated optical clock pulse generating circuit, a fully-optical modulator which is difficult to obtain and an accurate optical delayer which is difficult to be controlled are necessary.
FIG. 46 illustrates a technique for solving a phase balancing problem caused by fluctuation in a delay time to generate an optical pulse. However, in this technique, a control accuracy drops around an optimal phase shift amount. Further, when an optical signal power changes, it becomes impossible to measure a transmittancy rate in the optical modulator. Further, since two phase shift amounts are assumed from a transmittancy rate in an optical modulator as in the technique illustrated in FIG. 43, it is difficult to optimize the phase shift amount.
In HEI 8-111662, an effect of synchronizing a driving signal of the polarization scrambler and a data is discussed. However, a solution for disturbance in synchronization (a fluctuation of a transmission delay time of a fiber due to a temperature fluctuation, for example) is not disclosed. In HEI 8-111662, a circuit configuration for detecting a phase of a data signal inputted to the polarization scrambler and driving the polarization scrambler in an optimal synchronized phase is not disclosed.